NAME

xen -- Xen Hypervisor Guest (DomU) Support

SYNOPSIS

To compile para-virtualized (PV) Xen guest support into an i386 kernel,
place the following lines in your kernel configuration file:
optionsPAEoptionsXENnooptionsNATIVE
To compile hardware-assisted virtualization (HVM) Xen guest support with
para-virtualized drivers into an amd64 kernel, place the following lines
in your kernel configuration file:
optionsXENHVMdevicexenpci

DESCRIPTION

The Xen Hypervisor allows multiple virtual machines to be run on a single
computer system. When first released, Xen required that i386 kernels be
compiled "para-virtualized" as the x86 instruction set was not fully
virtualizable. Primarily, para-virtualization modifies the virtual
memory system to use hypervisor calls (hypercalls) rather than direct
hardware instructions to modify the TLB, although para-virtualized device
drivers were also required to access resources such as virtual network
interfaces and disk devices.
With later instruction set extensions from AMD and Intel to support fully
virtualizable instructions, unmodified virtual memory systems can also be
supported; this is referred to as hardware-assisted virtualization (HVM).
HVM configurations may either rely on transparently emulated hardware
peripherals, or para-virtualized drivers, which are aware of
virtualization, and hence able to optimize certain behaviors to improve
performance or semantics.
FreeBSD supports a fully para-virtualized (PV) kernel on the i386
architecture using optionsXEN and nooptionsNATIVE; currently, this
requires use of a PAE kernel, enabled via optionsPAE.
FreeBSD supports hardware-assisted virtualization (HVM) on both the i386
and amd64 kernels; however, PV device drivers with an HVM kernel are only
supported on the amd64 architecture, and require optionsXENHVM and
devicexenpci.
Para-virtualized device drivers are required in order to support certain
functionality, such as processing management requests, returning idle
physical memory pages to the hypervisor, etc.
XenDomUdevicedrivers
Xen para-virtualized drivers are automatically added to the kernel if a
PV kernel is compiled using optionsXEN; for HVM environments, optionsXENHVM and devicexenpci are required. The follow drivers are supported:
balloon Allow physical memory pages to be returned to the
hypervisor as a result of manual tuning or automatic
policy.
blkback Exports local block devices or files to other Xen domains
where they can then be imported via blkfront.
blkfront Import block devices from other Xen domains as local
block devices, to be used for file systems, swap, etc.
console Export the low-level system console via the Xen console
service.
control Process management operations from Domain 0, including
power off, reboot, suspend, crash, and halt requests.
evtchn Expose Xen events via the /dev/xen/evtchn special device.
netback Export local network interfaces to other Xen domains
where they can be imported via netfront.
netfront Import network interfaces from other Xen domains as local
network interfaces, which may be used for IPv4, IPv6,
etc.
pcifront Allow physical PCI devices to be passed through into a PV
domain.
xenpci Represents the Xen PCI device, an emulated PCI device
that is exposed to HVM domains. This device allows
detection of the Xen hypervisor, and provides interrupt
and shared memory services required to interact with the
hypervisor.
Performanceconsiderations
In general, PV drivers will perform better than emulated hardware, and
are the recommended configuration for HVM installations.
Using a hypervisor introduces a second layer of scheduling that may limit
the effectiveness of certain FreeBSD scheduling optimisations. Among
these is adaptive locking, which is no longer able to determine whether a
thread holding a lock is in execution. It is recommended that adaptive
locking be disabled when using Xen:
optionsNO_ADAPTIVE_MUTEXESoptionsNO_ADAPTIVE_RWLOCKSoptionsNO_ADAPTIVE_SX

BUGS

FreeBSD is only able to run as a Xen guest (DomU) and not as a Xen host
(Dom0).
A fully para-virtualized (PV) kernel is only supported on i386, and not
amd64.
Para-virtualized drivers under hardware-assisted virtualization (HVM)
kernel are only supported on amd64, not i386.
As of this release, Xen PV DomU support is not heavily tested;
instability has been reported during VM migration of PV kernels.
Certain PV driver features, such as the balloon driver, are under-
exercised.